sidedoor is a Backdoor using a reverse SSH tunnel on Debian/Ubuntu systems.
sidedoor maintains a reverse SSH tunnel to provide a backdoor. sidedoor can be used to remotely control a device behind a NAT. The sidedoor user has full root access configured in /etc/sudoers.d.

Howto:
1. Install sidedoor. For now, sudo dpkg -i sidedoor*.deb. You3. can build a package by running dpkg-buildpackage -us -uc -b.
2. Optionally, lock down the local SSH server by disabling password authentication (ChallengeResponseAuthentication no and PasswordAuthentication no) and listening only on localhost (ListenAddress ::1 and ListenAddress 127.0.0.1) in /etc/ssh/sshd_config. Then restart or reload sshd, e.g., sudo service ssh reload.
3. Configure REMOTE_SERVER and TUNNEL_PORT in /etc/default/sidedoor.
4. Install an SSH private key to access the remote server in /var/lib/sidedoor/.ssh/id_rsa. The corresponding public key will need to be included in the remote user’s ~/.ssh/authorized_keys file.
5. Install SSH public key(s) to control access to the local sidedoor user in /var/lib/sidedoor/.ssh/authorized_keys.
6. Restart sidedoor service, e.g., sudo service sidedoor restart.
7. Optionally, modify ssh_config_example and include it in a client’s ~/.ssh/config file to easily access the tunnelled backdoor.